1. Field of the Invention
The present invention relates to method and apparatus for cooling molded plastic pieces after the molding is finished. In particular, the present invention relates to method and apparatus for post mold cooling of plastic pieces wherein at least two cooling stations are provided to cool the interior of the plastic pieces. The present invention also relates to method and apparatus wherein pressurized sealing of each plastic piece is carried out at a cooling station such that the pressure seal does not contact the plastic piece. The present invention also relates to method and apparatus wherein plastic piece cavity tubes are provided with vents to prevent deformation of negative draft plastic pieces. Preferably, the plastic pieces comprise plastic preforms.
2. Related Art
A variety of post mold cooling methods are currently employed on injection molding machines (e.g., an Index machine platform) in various sequences to optimize the cooling of freshly molded plastic parts. Some parts (for example plastic preforms) are typically injection-molded using PET resin, and can have wall thicknesses varying from about 2.00 mm to greater than 4.00 mm, and consequently require extended cooling periods to solidify into defect-free parts. Heavy walled parts (such as these made from a material that has a high resistance to thermal heat transfer, like plastic resin) can exhibit a xe2x80x9creheatingxe2x80x9d phenomena that can produce defective parts after they have been ejected from the mold.
In the case of PET performs some of these manufacturing defects are:
Crystallinity: The resin recrystallizes due to the elevated temperature of the core resin not cooling quickly enough. The white appearance of the crystals impairs the clarity of the final product.
Surface blemishes: The ejected performs, initially having solidified surfaces are reheated by the core material which causes the surface to soften and be easily marred. Sometimes this surface reheating can be severe enough to cause touching parts to weld together.
Geometric inaccuracies: Handling partly-cooled performs or attempting to further cool them in devices that do not maintain their geometric shape while their surfaces are reheated can cause the preform""s round diameter to become oval shaped or the smooth surface to become wrinkled or non-linear.
The above-noted problems could be alleviated somewhat by extending the cooling time of the injection molded performs. However, this will cause the injection molding cycle to be lengthy, typically 25 seconds or longer, wherein the majority of this time was used solely for cooling purposes. In an effort to improve the production efficiency of this process, several techniques are employed to perform a xe2x80x9cpost mold coolingxe2x80x9d function, wherein partially-cooled performs are ejected from the injection mold after an initially cooled surface skin has formed to allow the part to be ejected without deformation. The partially-cooled performs are then handed off to a downstream device that continues to hold the perform while removing the remaining heat so that the preform can subsequently be handled without damage. Typically, the preform surface temperature needs to be lowered to about 72xc2x0 C. to ensure safe handling.
The early ejection of partially-cooled performs released the injection molding equipment earlier in the molding cycle, thereby significantly improving the production efficiency of the equipment. Injection molding cycle times typically were halved from 25 seconds to about 12 seconds or less in some instances depending on the perform design being molded.
Some examples of post mold cooling technology are shown in U.S. Pat. Nos. 4,729,732; Re. 33,237; 5,447,426; and 6,171,541, the contents of each being incorporated herein by reference.
Another approach to extending the cooling time for performs is to utilize a turret molding machine in which more than one set of injection molding cores are employed. An example is the Index machine, shown in U.S. Pat. Nos. 5,728,409; 5,830,404; 5,750,162; and 5,817,345 (the contents of each being incorporated herein by reference), which teach using a turret block having four faces and four core sets that are sequentially mated with one cavity set to hold the injection mold performs. Preforms molded on this kind of equipment can be produced in molding cycle times of typically 10-13 seconds.
A disadvantage of the above-described approach is the cost of the additional core side tooling that is required. In order to reduce this cost, Index machines with fewer core side tooling sets were employed. However, to maintain the cycle times, additional post mold cooling devices are needed to complete the perform cooling. Examples of Index machines with post mold cooling devices are shown in U.S. Pat. Nos. 6,059,557; 6,123,538; 6,143,225; and 6,113,834, the contents of each being incorporated herein by reference.
One technique for improving the rate of heat transfer from a cooling perform is to pressurize its interior volume while cooling it in a cavity. This method helps keep the preform""s exterior surface in contact with the cooling cavity surface, and counters the shrinkage of the perform which tends to separate the two surfaces. This allows good heat transfer to be maintained. Examples of pressurized perform cooling are shown in U.S. Pat. Nos. 4,950,152; and 5,679,306, and in EP 0 900 135, the contents of each being incorporated herein by reference.
Therefore, there is a need for post-mold cooling method and apparatus, which provides rapid, efficient cooling while further reducing the molding cycle time to further decrease the cost of producing molded plastic pieces.
It is an object of the present invention to provide post-mold cooling method and apparatus for efficiently cooling molded plastic pieces.
According to a first aspect of the present invention, structure and/or steps are provided for cooling a plurality of plastic parts whereby a take out structure/step is configured to hold the plurality of plastic parts oriented such that closed ends thereof are disposed toward an inside of the take out structure and open ends thereof are disposed toward an outside of the take out structure. A movement structure/step is configured to cause relative movement between the take out structure and at least one of a first cooling station and a second cooling station. The first cooling station includes a first cooling structure/step configured to provide a cooling fluid to an inside of the plurality of plastic parts through the open ends thereof. The second cooling station includes a second cooling structure/step configured to provide a cooling fluid to an inside of the plurality of plastic parts through the open ends thereof.
According to a second aspect of the present invention, structure and/or steps are provided for post-mold cooling of a matrix of plastic preforms whereby a take out plate includes a first plurality of receiving tubes configured to receive a first plurality of plastic preforms, and a second plurality of receiving tubes configured to receive a second plurality of plastic preforms. At least one of the first and second plurality of receiving tubes is configured to cool outside surfaces of the corresponding plurality of plastic preforms. A movement structure/step is configured to provide relative movement between the take out plate and first and second cooling stations. The first cooling station includes (i) a plurality of injector devices, each of which is configured to inject a pressurized cooling medium to an interior of a corresponding plastic preforms, and (ii) a plurality of sealing devices, each of which is configured to provide a pressure seal between the injected pressurized cooling medium and a lower, ambient pressure. The second cooling station includes (i) a plurality of cooling pins, each of which is configured to direct a cooling medium at a tip of an inside of a corresponding plastic preform so that the directed cooling medium flows down an inside surface of the corresponding plastic preform and is exhausted to an outside of the take out plate. A control structure/step causes the movement structure/step to move the plurality of plastic preforms from the first cooling station to the second cooling station.
According to a third aspect of the present invention, plastic injection molding structure and/or steps include a plastic molding unit/process having a plurality of mold cavities and a plurality of carriers configured to provide a plurality of plastic parts. A take out device/process removes the plurality of plastic parts from the plastic molding unit/process. A movement structure/process is configured to cause relative movement between the take out device/process and at least one of a first cooling station and a second cooling station. The first cooling station includes a first cooling structure/process configured to seal a pressurized cooling fluid on an interior of each of the plurality of plastic parts. The second cooling station includes a second cooling structure/process configured to provide a cooling fluid to an inside of the plurality of plastic parts.
According to a fourth aspect of the present invention, structure and/or steps are provided for post-molding cooling a plurality of plastic parts whereby a take out stage takes a plurality of plastic parts from a molding machine, the take out stage including a plurality of receiver tubes adapted to receive the plurality of plastic parts such that an open end of each preform is exposed. A plurality of cooling stations are coupled to the take out stage, each cooling station having a plurality of cooling pins for injecting a cooling fluid into an interior of corresponding plastic parts through the exposed ends.
According to a fifth aspect of the present invention, structure and/or steps are provided for a plastic preform cooling station including a holder for holding a plurality of plastic parts such that their open ends are exposed. A plurality of cooling pins are provided for injecting a cooling medium into the exposed ends of the plurality of plastic parts. Sealing structure is provided for sealing the exposed ends of the plurality of plastic parts from ambient pressure. A controller is provided for causing the sealing structure and the plurality of cooling pins to inject cooling air into the plurality of plastic parts and maintain pressure therein higher than ambient pressure, and then to release the pressure.
According to a sixth aspect of the present invention, structure and/or steps are provided for cooling a plastic preform, including a receiving tube which receives the plastic preform such that an open end of the plastic preform is exposed, the receiving tube being in contact with an outside surface of the plastic preform. A cooling pin is inserted through the open end of the plastic preform into an interior thereof, and injects a cooling fluid therein. A pressure seal is coupled between the receiving tube and the cooling pin to maintain a pressure inside the plastic preform higher than ambient pressure. The pressure seal is configured to permit fluid communication between the inside of the plastic preform and a portion of the outside of the plastic preform.
According to a seventh aspect of the present invention, structure and/or steps are provided for cooling a plastic preform, including a receiving tube which receives the plastic preform such that an open end of the plastic preform is exposed, the receiving tube being in contact with an outside surface of the plastic preform. A cooling pin is inserted through the open end of the plastic preform into an interior thereof, and injects a cooling fluid therein. A pressure seal is coupled between the receiving tube and the cooling pin to maintain a pressure inside the plastic preform higher than ambient pressure. A vent is provided in the receiving tube to permit fluid communication between the inside of the plastic preform and an outside of said receiving tube.
According to an eighth aspect of the present invention, structure and/or steps are provided for cooling a molded plastic part, including a receiver cavity configured to hold the plastic part such that an opening in the plastic part is exposed. A cooling fluid provider is configured to provide a cooling fluid to an inside portion of the plastic part through the opening in the plastic part. A pressure seal is configured to provide a pressure seal between the inside of the plastic part and ambient pressure. A controller is configured to control the cooling fluid provider and the pressure seal to cause: (i) the pressure seal to provide the pressure seal and said cooling fluid provider to provide the cooling fluid such that a higher than ambient pressure is maintained on the inside portion of the plastic part; (ii) the pressure seal to release the pressure such that the pressure inside the plastic part is reduced; and (iii) the cooling fluid provider to provide further cooling fluid after the pressure seal releases the pressure such that the further cooling fluid flushes the inside of the plastic part.
According to a ninth aspect of the present invention, structure and/or steps are provided for cooling a molded plastic part, includes structure and/or steps for: (i) holding a plastic part in a receiver such that an opening of the plastic part is exposed; (ii) positioning a cooling fluid device to provide a cooling fluid to an interior of the plastic part through the opening; (iii) sealing the interior of the plastic part from ambient pressure; (iv) pressurizing the interior of the plastic part with the cooling fluid to higher than ambient pressure; (v) depressurizing the interior of the plastic part to ambient pressure; (vi) flushing the interior of the plastic part with cooling fluid; (vii) re pressurizing the interior of the plastic part with the cooling fluid to higher than ambient pressure; (viii) again depressurizing the interior of the plastic part to ambient pressure; and (ix) again flushing the interior of the plastic part with cooling fluid.